Coated Narrow Band Green Phosphor

ABSTRACT

A coated phosphor may comprise: phosphor particles comprised of a phosphor with composition (M)(A)2S4:Eu, wherein: M is at least one of Mg, Ca, Sr and Ba; and A is at least one of Ga, Al, In, Y; a dense impermeable (pinhole-free) coating of an oxide material encapsulating individual ones of the phosphor particles. The coated phosphor is configured to satisfy one or more of the conditions: (1) under excitation by blue light, the reduction in photoluminescent intensity at the peak emission wavelength after 1,000 hours of aging at about 85° C. and about 85% relative humidity is no greater than about 30%; (2) the change in chromaticity coordinates CIE(y), ΔCIE y, after 1,000 hours of aging at about 85° C. and about 85 % relative humidity is less than about 5×10−3; (3) said coated phosphor does not turn black when suspended in a 1 mol/L silver nitrate solution for at least two hours at 85° C.; (4) said coated phosphor does not turn black when suspended in a 1 mol/L silver nitrate solution for at least one day at 20° C.; and (5) said coated phosphor does not turn black when suspended in a 1 mol/L silver nitrate solution for at least 5 days at 20° C.

FIELD OF THE INVENTION

Embodiments of the invention are directed to narrow band green andcoated narrow band green phosphors with general composition based onMA₂S₄:Eu, wherein: M comprises at least one of Mg, Ca, Sr and Ba, Acomprises at least one of Ga, Al, In, La and Y, and the coating is oneor more oxides chosen from the group of materials including aluminumoxide, silicon oxide, titanium oxide, zinc oxide, magnesium oxide,zirconium oxide and chromium oxide, and light emitting devices includingthe same.

BACKGROUND OF THE INVENTION

MGa₂S₄:Eu materials, where M comprises at least one of Mg, Ca, Sr andBa, show green color emission around 535 nm, and provide high lumenefficacy of LED lighting. However, the narrow band green phosphors withgeneral composition MGa₂S₄:Eu are hygroscopic, and exhibit rapiddeterioration of photoluminescence due to exposure to moisture (watervapor), oxygen and/or heat. There is a need for narrow band greenphosphors with general composition MGa₂S₄:Eu, with coatings which areeffective at protecting the phosphor particles from moisture and oxygenand enable a commercially useful phosphor.

Furthermore, there is a need for new compositions of narrow band greenphosphors based on the phosphor with general composition MA₂S₄:Eu,wherein: M comprises at least one of Mg, Ca, Sr and Ba, A comprises atleast one of Ga, Al, In, La and Y, providing improved lumen maintenanceand stable chromaticity over the lifetime of a light emitting device.

SUMMARY OF THE INVENTION

Embodiments of the invention concern narrow band green phosphors withgeneral composition based on MA₂S₄:Eu, wherein: M comprises at least oneof Mg, Ca, Sr and Ba, A comprises at least one of Ga, Al, In, La and Y,having a dense impermeable (i.e. pinhole-free) coating of an oxidematerial encapsulating individual ones of phosphor particles.

According to a first aspect of the present invention, there is provideda coated phosphor comprises phosphor particles comprised of a phosphormaterial with composition (M)(A)₂S₄:Eu, wherein: M is at least one ofMg, Ca, Sr and Ba; and A is at least one of Ga, Al, In, Y; and a denseimpermeable coating of an oxide material encapsulating individual onesof said phosphor particles; and wherein said coated phosphor isconfigured to satisfy at least one of the conditions: (1) underexcitation by blue light, the reduction in photoluminescent intensity atthe peak emission wavelength after 1,000 hours of aging at about 85° C.and about 85% relative humidity is no greater than about 30%; (2) thechange in chromaticity coordinates CIE(y), ΔCIE y, after 1,000 hours ofaging at about 85° C. and about 85% relative humidity is less than about5×10⁻³; (3) said coated phosphor does not turn black when suspended in a1 mol/L silver nitrate solution for at least two hours at 85° C.; (4)said coated phosphor does not turn black when suspended in a 1 mol/Lsilver nitrate solution for at least one day at 20° C.; and (5) saidcoated phosphor does not turn black when suspended in a 1 mol/L silvernitrate solution for at least 5 days at 20° C.

The coated phosphor may be configured such that the change inchromaticity coordinates CIE(y), ΔCIE y, after 1,000 hours of aging atabout 85° C. and about 85% relative humidity is less than or equal toabout 3×10⁻³.

The oxide may be one or more materials chosen from the group comprisingaluminum oxide, silicon oxide, titanium oxide, zinc oxide, magnesiumoxide, zirconium oxide and chromium oxide.

In some embodiments, the oxide material may be alumina.

The coating may have a thickness in the range of 100 nm to 5 μm or inthe range of 800 nm to 1.2 μm.

The phosphor particles may have a size distribution having a D₅₀ valuewithin the range of 5 μm to 15 μm.

In some embodiments, M may be Sr.

In some embodiments, A may be Ga.

In some embodiments, A may be Ga, M may be Sr and said oxide materialmay be alumina.

The coated phosphor may have a peak photoluminescence between 535 nm and537 nm and a FWHM of between 48 nm and 50 nm, when excited by blue lightwith a peak emission of about 450 nm.

The phosphor material composition in accordance with embodimentsdescribed herein may further comprise Praseodymium (Pr). The Pr may actas a co-activator and the inventors have discovered that this canimprove the quantum efficiency of the phosphor and increase emissionintensity by 5-8%. The co-activation of a narrow band green phosphor ofcomposition (M)(A)₂S₄ with Eu and Pr is believed to be inventive in itsown right. Thus, according to some embodiments, a coated phosphor maycomprise phosphor particles comprised of a phosphor material withcomposition (M)(A)₂S₄:Eu; Pr, wherein: M is at least one of Mg, Ca, Srand Ba; and A is at least one of Ga, Al, In, Y; a dense impermeablecoating of an oxide material encapsulating individual ones of saidphosphor particles. In some embodiments, the coated phosphor maycomprise particles phosphor particles comprised of a phosphor materialwith composition SrGa₂S₄:Eu; Pr having a dense impermeable coating ofalumina encapsulating individual ones of said phosphor particles.

In another aspect, the present invention encompasses a method of forminga coated phosphor, comprising: providing phosphor particles comprised ofa phosphor material with composition (M)(A)₂S₄:Eu wherein: M is at leastone of Mg, Ca, Sr and Ba; and A is at least one of Ga, Al, In, Y; anddepositing a dense impermeable coating of an oxide materialencapsulating individual ones of said phosphor particles by a gas phaseprocess in a fluidized bed reactor; wherein said coated phosphor isconfigured to satisfy at least one of the conditions: (1) underexcitation by blue light, the reduction in photoluminescent intensity atthe peak emission wavelength after 1,000 hours of aging at about 85° C.and about 85% relative humidity is no greater than about 30%; (2) thechange in chromaticity coordinates CIE(y), ΔCIE y, after 1,000 hours ofaging at about 85° C. and about 85% relative humidity is less than about5×10⁻³; (3) said coated phosphor does not turn black when suspended in a1 mol/L silver nitrate solution for at least two hours at 85° C.; (4)said coated phosphor does not turn black when suspended in a 1 mol/Lsilver nitrate solution for at least one day at 20° C.; and (5) saidcoated phosphor does not turn black when suspended in a 1 mol/L silvernitrate solution for at least 5 days at 20° C. Of course, it will beunderstood that the method encompasses forming a coated phosphor whichmay comprise phosphor particles comprised of a phosphor material withcomposition (M)(A)₂S₄:Eu; Pr in the same manner as that described above.

It will be appreciated that the method of forming a coated phosphorencompasses forming the various coated phosphors described herein.

In a further aspect, the present invention envisages a white lightemitting device comprising: an excitation source with a dominantemission wavelength within a range from 200 nm to 480 nm; a coatedphosphor as described herein, with a first phosphor peak emissionwavelength; and a second phosphor with a second phosphor peak emissionwavelength different to said first phosphor peak wavelength.

In some embodiments, the coated phosphor may absorb radiation at awavelength of 450 nm and emits light with a photoluminescence peakemission wavelength between about 530 nm and about 545 nm; and saidsecond phosphor emits light with a photoluminescence peak emissionwavelength between about 600 nm and about 650 nm. Such white lightemitting devices may find applications for display backlightingapplications and general lighting.

For backlighting applications, the excitation source may have a dominantemission wavelength within a range from 440 nm to 480 nm; and the whitelight emitting device may have an emission spectrum with clearlyseparated blue, green and red peaks, and a color gamut after LCD RGBcolor filters of at least 85% of NTSC.

In yet another aspect of the present invention, there is provided awhite light emitting device for backlighting, comprising: an excitationsource with a dominant emission wavelength within a range from 440 nm to480 nm; a coated phosphor as described herein with a first phosphor peakemission wavelength between about 530 nm and about 545 nm; and a secondphosphor with a second phosphor peak emission wavelength different tosaid first phosphor peak wavelength, said second phosphor peak emissionwavelength being between about 600 nm and about 650 nm; wherein saidwhite light emission device has an emission spectrum with clearlyseparated blue, green and red peaks, and a color gamut after LCD RGBcolor filters of at least 85% of NTSC.

It various white light emitting devices, it may be that at least one ofthe coated phosphor and second phosphor is located in a remote phosphorcomponent. For example, in some embodiments, the coated phosphor may belocated in the remote phosphor component and the second phosphor may belocated in a package housing the excitation source. It may be that, forexample, the remote phosphor component comprises a film such as“on-film” that is applied to the rear surface of the display.

In yet further embodiments, the narrow band green phosphor can furthercomprise elements M′ and A′; where M′ comprises for example Li, Na andK; and A′ comprises for example Si, Ge and Ti.

In some such embodiments, the present invention contemplates a coatedphosphor comprising phosphor particles comprised of a phosphor materialwith composition (M,M′)(A,A′)₂S₄:Eu, wherein: M is at least one of Mg,Ca, Sr and Ba; M′ is at least one of Li, Na and K; A is at least one ofGa, Al, In, La and Y; A′ is at least one of Si, Ge and Ti; and a denseimpermeable coating of an oxide material encapsulating individual onesof said phosphor particles; and wherein said coated phosphor isconfigured to satisfy at least one of the conditions: (1) underexcitation by blue light, the reduction in photoluminescent intensity atthe peak emission wavelength after 1,000 hours of aging at about 85° C.and about 85% relative humidity is no greater than about 30%; (2) thechange in chromaticity coordinates CIE(y), ΔCIE y, after 1,000 hours ofaging at about 85° C. and about 85% relative humidity is less than about5×10⁻³; (3) said coated phosphor does not turn black when suspended in a1 mol/L silver nitrate solution for at least two hours at 85° C.; (4)said coated phosphor does not turn black when suspended in a 1 mol/Lsilver nitrate solution for at least one day at 20° C.; and (5) saidcoated phosphor does not turn black when suspended in a 1 mol/L silvernitrate solution for at least 5 days at 20° C. In such phosphormaterials, it is believed that M′ substitutes for M, and A′ substitutesfor A in the MA₂S₄ crystalline lattice.

In another aspect, the present invention envisages a coated phosphorcomprising phosphor particles comprised of a phosphor material withcomposition (M)(A)₂S₄:Eu, M′, A′ wherein: M is at least one of Mg, Ca,Sr and Ba; M′ is at least one of Li, Na and K; A is at least one of Ga,Al, and In; and A′ is at least one of Si, Ge, La, Y and Ti; and a denseimpermeable coating of an oxide material encapsulating individual onesof said phosphor particles; and wherein said coated phosphor isconfigured to satisfy at least one of the conditions: (1) underexcitation by blue light, the reduction in photoluminescent intensity atthe peak emission wavelength after 1,000 hours of aging at about 85° C.and about 85% relative humidity is no greater than about 30%; (2) thechange in chromaticity coordinates CIE(y), ΔCIE y, after 1,000 hours ofaging at about 85° C. and about 85% relative humidity is less than about5×10⁻³; (3) said coated phosphor does not turn black when suspended in a1 mol/L silver nitrate solution for at least two hours at 85° C.; (4)said coated phosphor does not turn black when suspended in a 1 mol/Lsilver nitrate solution for at least one day at 20° C.; and (5) saidcoated phosphor does not turn black when suspended in a 1 mol/L silvernitrate solution for at least 5 days at 20° C.

The coated phosphor may be configured such that the change inchromaticity coordinates CIE(y), ΔCIE y, after 1,000 hours of aging atabout 85° C. and about 85% relative humidity is less than or equal toabout 3×10⁻³.

The oxide may be one or more materials chosen from the group comprisingaluminum oxide, silicon oxide, titanium oxide, zinc oxide, magnesiumoxide, zirconium oxide and chromium oxide.

In some embodiments, the oxide material may be alumina.

The coating may have a thickness in the range of 100 nm to 5 μm or inthe range of 800 nm to 1.2 μm.

The phosphor particles may have a size distribution having a D₅₀ valuewithin the range of 5 μm to 15 μm.

In some embodiments, M may be Sr.

In some embodiments, A may be Ga.

In some embodiments, A may be Ga, M may be Sr and said oxide materialmay be alumina.

The coated phosphor may have a peak photoluminescence between 535 nm and537 nm and a FWHM of between 48 nm and 50 nm, when excited by blue lightwith a peak emission of about 450 nm.

In a further aspect of the present invention, there is provided aphosphor comprising phosphor particles comprised of a phosphor materialwith composition—(M_(1-x)Li_(x))(A_(1-x/2)Si_(x/2))₂S₄:Eu, wherein: M isat least one of Mg, Ca, Sr and Ba; and A is at least one of Ga, Al, In,La and Y; wherein 0<x<0.1. The phosphor may further comprise a denseimpermeable coating of an oxide material encapsulating individual onesof said phosphor particles. The phosphor may have a coating have athickness in the range of 100 nm to 5 μm or in the range of 800 nm to1.2 μm. The phosphor particles may be characterized by a sizedistribution having a D₅₀ value within the range of 5 μm to 15 μm. Theoxide material may be one or more materials chosen from the groupcomprising aluminum oxide, silicon oxide, titanium oxide, zinc oxide,magnesium oxide, zirconium oxide and chromium oxide. The oxide materialmay be alumina. In some embodiments, M may be Sr and A may be Ga. Thephosphor phosphor may have a peak photoluminescence between 535 nm and537 nm and a FWHM of between 48 nm and 50 nm, when excited by a bluelight source with a dominant emission wavelength of about 450 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIGS. 1 & 2 shows XRD data for NBG (SrGa₂S₄:Eu) phosphor and Li/Si dopedNBG phosphor, according to some embodiments of the invention;

FIG. 3 shows normalized emission spectra for NBG phosphor and Li/10% Sidoped NBG phosphor, according to some embodiments of the invention;

FIG. 4 shows normalized peak emission versus temperature for NBGphosphor and NBG phosphor doped with various amounts of Li/Si, accordingto some embodiments of the invention;

FIG. 5 is a schematic representation of a phosphor particle coatingapparatus, according to some embodiments of the invention;

FIGS. 6A-6E are SEM micrographs of alumina coated NBG phosphorparticles, according to an embodiment of the invention;

FIGS. 7A-7D are SEM micrographs of M535 coated alkaline earth metalthiogallate phosphor particles;

FIG. 8 shows reliability data, relative photoluminescence intensityversus time, for an LED (SMD 7020 LED module 450.0-452.5 nm) operatedunder accelerated testing conditions WHTOL 85° C./85% RH for (i) aluminacoated NBG phosphor, according to an embodiment of the invention, (ii)M535 phosphor, and (iii) β-SiAlON:Eu (540 nm) phosphor;

FIG. 9 shows reliability data, change of chromaticity ΔCIE x versustime, for an LED (SMD 7020 LED module 450.0-452.5 nm) operated underaccelerated testing conditions WHTOL 85° C./85% RH for (i) aluminacoated NBG phosphor, according to an embodiment of the invention, (ii)M535 phosphor, and (iii) β-SiAlON:Eu (540 nm) phosphor;

FIG. 10 shows reliability data, change of chromaticity ΔCIE y versustime, for an LED (SMD 7020 LED module 450.0-452.5 nm) operated underaccelerated testing conditions WHTOL 85° C./85% RH for (i) aluminacoated NBG phosphor, according to an embodiment of the invention, (ii)M535 phosphor, and (iii) β-SiAlON:Eu (540 nm) phosphor;

FIGS. 11A—11C are SEM micrographs of multi—layer coated(alumina—aluminum oxide—Al₂O₃, titania—titanium dioxide—TiO₂) NBGphosphor particles, according to an embodiment of the invention;

FIG. 12 shows reliability data, relative photoluminescence intensityversus time, for a white light emitting device (SMD 7020 LED module)operated under accelerated testing conditions WHTOL 85° C./85% RH for ablue LED (450.0-452.5 nm) combined with (i) alumina coated NBG phosphorand CSS red phosphor (alumina coated CaSe_(1-x)S_(x):Eu; 630 nm),according to an embodiment of the invention, and (ii) β-SiAlON:Euphosphor (540 nm) and CSS red phosphor (630 nm);

FIG. 13 shows reliability data, change of chromaticity ΔCIE x versustime, for a white light emitting device (SMD 7020 LED module) operatedunder accelerated testing conditions WHTOL 85° C./85% RH for a blue LED(450.0-452.5 nm) combined with (i) alumina coated NBG phosphor and CSSred phosphor (630 nm) according to an embodiment of the invention, and(ii) β-SiAlON:Eu (540 nm) phosphor and CSS red phosphor (630 nm);

FIG. 14 shows reliability data, change of chromaticity ΔCIE y versustime, for a white light emitting device (SMD 7020 LED module) operatedunder accelerated testing conditions WHTOL 85° C./85% RH for a blue LED(450.0-452.5 nm) combined with (i) alumina coated NBG phosphor and CSSred phosphor according to an embodiment of the invention, and (ii)β-SiAlON:Eu (540 nm) phosphor and CSS red phosphor (630 nm);

FIG. 15 shows reliability data, relative photoluminescence intensityversus time, for a white light emitting device (SMD 7020 LED module)operated under accelerated testing conditions WHTOL 60° C./90% RH for ablue LED (450.0-452.5 nm) combined with (i) alumina coated NBG phosphorand KSF (K₂SiF₆:Mn) red phosphor, according to an embodiment of theinvention, ii) alumina coated NBG phosphor and CSS red phosphor (630nm), according to an embodiment of the invention, iii) β-SiAlON:Eu (540nm) phosphor and KSF red phosphor, and (iv) β-SiAlON:Eu (540 nm)phosphor and CSS red phosphor (630 nm);

FIG. 16 shows reliability data, change of chromaticity ΔCIE x versustime, for a white light emitting device (SMD 7020 LED module) operatedunder accelerated testing conditions WHTOL 60° C./90% RH for a blue LED(450.0-452.5 nm) combined with (i) alumina coated NBG phosphor and KSFred phosphor, according to an embodiment of the invention, (ii) aluminacoated NBG phosphor and CSS red phosphor (630 nm) according to anembodiment of the invention, (iii) β-SiAlON:Eu (540 nm) phosphor and KSFred phosphor, and (iv) β-SiAlON:Eu (540 nm) phosphor and CSS redphosphor (630 nm);

FIG. 17 shows reliability data, change of chromaticity ΔCIE y versustime, for a white light emitting device (SMD 7020 LED module) operatedunder accelerated testing conditions WHTOL 60° C./90% RH for a blue LED(450.0-452.5 nm) combined with (i) alumina coated NBG phosphor and KSFred phosphor, according to an embodiment of the invention, (ii) aluminacoated NBG phosphor and CSS red phosphor (630 nm) according to anembodiment of the invention, (iii) β-SiAlON:Eu (540 nm) phosphor and KSFred phosphor, and (iv) β-SiAlON:Eu (540 nm) phosphor and CSS redphosphor (630 nm);

FIG. 18 shows a cross-sectional schematic representation of a whitelight emitting device, according to some embodiments of the invention;

FIG. 19 shows a white light emission spectra of a 3000K white lightemitting device comprising alumina coated NBG phosphor and CSS redphosphor, according to some embodiments of the invention;

FIG. 20 shows the filtering characteristics, light transmission versuswavelength, for red, green and blue filter elements (Hisense) of an LCDdisplay;

FIG. 21 shows a white light emission spectra of a white light emittingdevice comprising alumina coated NBG (537 nm) phosphor and KSF redphosphor, according to some embodiments of the invention;

FIG. 22 shows a white light emission spectra of a white light emittingdevice comprising alumina coated NBG (537 nm) phosphor and KSF redphosphor, according to some embodiments of the invention, afterfiltering by the red, green and blue filter elements (Hisense);

FIG. 23 shows the filtering characteristics, light transmission versuswavelength, for red, green and blue filter elements (AUO) of an LCDdisplay;

FIG. 24 shows a white light emission spectra of a white light emittingdevice comprising alumina coated NBG (537 nm) phosphor and KSF redphosphor, according to some embodiments, after filtering by the red,green and blue filter elements (AUO) of an LCD display;

FIG. 25 shows the 1931 CIE color coordinates of the NTSC standard andthe calculated RGB color coordinates from the white light source forwhich a spectrum is shown in FIG. 21, according to some embodiments ofthe invention; and

FIGS. 26A & 26B show a cut-away top view and a cross-sectional view,respectively, of a white light remote phosphor solid-state lightemitting device, according to some embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described in detailwith reference to the drawings, which are provided as illustrativeexamples of the invention so as to enable those skilled in the art topractice the invention. Notably, the figures and examples below are notmeant to limit the scope of the present invention to a singleembodiment, but other embodiments are possible by way of interchange ofsome or all of the described or illustrated elements. Moreover, wherecertain elements of the present invention can be partially or fullyimplemented using known components, only those portions of such knowncomponents that are necessary for an understanding of the presentinvention will be described, and detailed descriptions of other portionsof such known components will be omitted so as not to obscure theinvention. In the present specification, an embodiment showing asingular component should not be considered limiting; rather, theinvention is intended to encompass other embodiments including aplurality of the same component, and vice-versa, unless explicitlystated otherwise herein. Moreover, applicants do not intend for any termin the specification or claims to be ascribed an uncommon or specialmeaning unless explicitly set forth as such. Further, the presentinvention encompasses present and future known equivalents to the knowncomponents referred to herein by way of illustration.

Embodiments of the present invention are directed generally to narrowband green and coated narrow band green phosphors with generalcomposition based on MA₂S₄:Eu, wherein: M is at least one of Mg, Ca, Srand Ba, A is at least one of Ga, Al, In, La and Y, and the coating isone or more oxides chosen from the group of materials comprisingaluminum oxide, silicon oxide, titanium oxide, zinc oxide, magnesiumoxide, zirconium oxide and chromium oxide. For example, a narrow bandgreen phosphor may have a composition (M)(A)₂S₄:Eu, M′, A′ wherein: M isat least one of Mg, Ca, Sr and Ba; M′ is at least one of Li, Na and K; Ais at least one of Ga, Al, and In; and A′ is at least one of Si, Ge, La,Y and Ti. In the latter formula the dopants Eu, M′and A′ may be presentin substitutional sites, although other options for incorporation areenvisaged, such as interstitial sites. Furthermore, a narrow band greenphosphor may have a composition (M,M′)(A,A′)₂S₄:Eu, wherein: M is atleast one of Mg, Ca, Sr and Ba; M′ is at least one of Li, Na and K; A isat least one of Ga, Al, In, La and Y; and A′ is at least one of Si, Geand Ti; wherein M′ substitutes for M, and A′ substitutes for A in theMA₂S₄ crystalline lattice. In the latter formula the specificsubstitution sites are identified, although it is envisaged thatalternative substitutional sites may exist; for example, the inventorsenvisage that for doping with Li and Si the following structure mayprovide an alternative substitutional site for the Li:Sr(Ga_(1-2x)Si_(x)Li_(2x))₂S₄:Eu, wherein: M is at least one of Mg, Ca,Sr and Ba; and A is at least one of Ga, Al, In, La and Y; wherein0<x<0.1

Phosphor Synthesis

Synthesis of a doped SrGa₂S₄:Eu phosphor was by one of the followingprocesses.

In a first process, Ga₂S₃ material was synthesized by exposing a Ga₂O₃precursor to CS₂ in a N₂ atmosphere, at 900° C. for 6 hours. In order tosynthesize Li and Si doped Eu_(0.05)Sr_(0.95)Ga₂S₄ phosphor, EuCl₃,SrCO₃, Ga₂S₃, SiO₂ and Li₂CO₃ powders were weighed with molar ratio of0.05, 0.95, 1.0, 0.1 and 0.1. The powders were mixed using a mortar andpestle, and placed in an alumina crucible and calcined in a box-typefurnace at 850° C. for 4 hours followed by a further 2 hours at 900° C.exposed to a N₂ gas stream carrying CS₂. The resulting sintered phosphorcake was removed from the furnace and mortared, washed, and sieved priorto evaluation. This first process was used to synthesize the uncoatedphosphors described in Tables 1-16.

In a second process, which is particularly suitable for larger scalesynthesis of SrGa₂S₄:Eu, Pr phosphor for coating in a fluidized bedchemical vapor deposition reactor, a Ga₂O₃—SrSO₄—Eu₂O₃—Pr₂O₃ precursoris prepared as follows. 40 g of gallium metal was dissolved in 250 ml ofconcentrated nitric acid (70% HNO₃) in a 1 liter flask (approximately 4days at room temperature) to form Ga(NO₃)₃ solution. Then 1.4 g Eu₂O₃and 0.007 g Pr₆O₁₁ were dissolved in the acidic Ga(NO₃)₃ solution, andthe Ga(NO₃)₃—Eu(NO₃)₃—Pr(NO₃)₃ solution was transferred into a 3 literglass beaker. 30 g of powder (5-15 micron particle size) of SrSO₄ orSrCO₃ was dispersed in the nitrate solution while stirring, and thesolution was stirred for 1 hour at room temperature. The pH of thesolution was slowly adjusted to pH 7.0 by adding NH₄OH (5.0 wt %),resulting in precipitation of phosphor particles. After the precipitatessettled, the phosphor particles were filtered out and dried. Note thatthe above process may also be used to form the NBG phosphor with onlyone activator—Eu—if desired; furthermore, if Pr co-activator is used itwill typically be incorporated in amounts in the range of 0<Pr<0.15molar percent of Eu. This second process was used to synthesize thephosphors which were then coated in a fluidized bed chemical vapordeposition reactor as described below; data for these phosphors isprovided in Tables 17-21.

The first and second processes were also used to make NBG phosphors(SrGa₂S₄:Eu and related materials, as described herein) includingvarious dopants and/or substituted elements, as will be clear fromTables 1-6 below.

TABLE 1 Starting materials for manufacturing Li/Si doped SrGa₂S₄:Eu (5%Eu) EuCl₃ SrS Ga₂S₃ SiO₂ Li₂CO₃ Al₂O₃ Composition (mole) (mole) (mole)(mole) (mole) (mole) SrGa₂S₄:Eu 0.05 0.95 1.00 — — — SrGa₂S₄:Eu (5% Si)0.05 0.95 1.00 0.10 — — SrGa₂S₄:Eu (Li/2.5% Si) 0.05 0.95 1.00 0.05 0.05— SrGa₂S₄:Eu (Li/5% Si) 0.05 0.95 1.00 0.10 0.10 — SrGa₂S₄:Eu (Li/7.5%Si) 0.05 0.95 1.00 0.15 0.15 — SrGa₂S₄:Eu (Li/10% Si) 0.05 0.95 1.000.20 0.20 — SrGa₂S₄:Eu (Li/5% Si, 2.5% Al) 0.05 0.95 1.00 0.10 0.100.025 SrGa₂S₄:Eu (Li/5% Si, 5% Al) 0.05 0.95 1.00 0.10 0.10 0.050SrGa₂S₄:Eu (Li/5% Si, 10% Al) 0.05 0.95 1.00 0.10 0.10 0.100

TABLE 2 Starting materials for manufacturing In, Li/Si and Li, Y dopedSrGa₂S₄:Eu (5% Eu) EuCl SrS Ga₂S₃ SiO₂ Li₂CO₃ In₂O₃ Y₂O₃ Composition(mole) (mole) (mole) (mole) (mole) (mole) (mole) SrGa₂S₄:Eu (2.5% In,Li/2.5% 0.05 0.95 0.97 0.05 0.05 0.025 — SrGa₂S₄:Eu (5% Li, 5% Y) 0.050.95 0.95 — 0.1 — 0.05

TABLE 3 Starting materials for manufacturing Li/5% Si doped SrGa₂S₄:Euwith differing Eu concentrations EuCl₃ SrS Ga₂S₃ SiO₂ Li₂CO₃ Composition(mole) (mole) (mole) (mole) (mole) SrGa₂S₄:Eu (1% Eu, Li/5% Si) 0.010.95 1.00 0.10 0.10 SrGa₂S₄:Eu (2% Eu, Li/5% Si) 0.02 0.95 1.00 0.100.10 SrGa₂S₄:Eu (3% Eu, Li/5% Si) 0.03 0.95 1.00 0.10 0.10 SrGa₂S₄:Eu(4% Eu, Li/5% Si) 0.04 0.95 1.00 0.10 0.10 SrGa₂S₄:Eu (5% Eu, Li/5% Si)0.05 0.95 1.00 0.10 0.10

TABLE 4 Starting materials for manufacturing Na/Si doped SrGa₂S₄:Eu (5%Eu) EuCl₃ SrS Ga₂S₃ SiO₂ Na₂CO₃ Al₂O₃ Composition (mole) (mole) (mole)(mole) (mole) (mole) SrGa₂S₄:Eu 0.05 0.95 1.00 0.10 0.10 — (Na/5% Si)

TABLE 5 Starting materials for manufacturing Ca and Ba doped SrGa₂S₄:EuEuCl₃ SrS Ga₂S₃ CaS BaS Al₂O₃ Composition (mole) (mole) (mole) (mole)(mole) (mole) SrGa₂S₄:Eu 0.05 0.93 1.00 0.02 — — (5% Eu, 2% Ca)SrGa₂S₄:Eu 0.05 0.93 0.95 0.02 — 0.05 (5% Eu, 2% Ca, 5% Al) SrGa₂S₄:Eu0.01 0.93 0.90 0.02 — 0.10 (1% Eu, 2% Ca, 10% Al) SrGa₂S₄:Eu 0.02 0.930.90 0.02 — 0.10 (2% Eu, 2% Ca, 10% Al) SrGa₂S₄:Eu 0.03 0.93 0.90 0.02 —0.10 (3% Eu, 2% Ca, 10% Al) SrGa₂S₄:Eu 0.04 0.93 0.90 0.02 — 0.10 (4%Eu, 2% Ca, 10% Al) SrGa₂S₄:Eu 0.05 0.93 0.90 0.02 — 0.10 (5% Eu, 2% Ca,10% Al) SrGa₂S₄:Eu 0.05 0.89 1.00 0.04 0.02 — (4% Ca, 2% Ba) SrGa₂S₄:Eu0.05 0.93 1.00 — 0.02 — (2% Ba)

TABLE 6 Starting materials for manufacturing In, La, Y and Al dopedSrGa₂S₄:Eu (5% Eu) EuCl₃ SrS Ga₂S₃ In₂S₃ La₂O₃ Y₂O₃ Al₂O₃ Composition(mole) (mole) (mole) (mole) (mole) (mole) (mole) SrGa₂S₄:Eu (2.5% In)0.05 0.95 0.975 0.025 — — — SrGa₂S₄:Eu (5% In) 0.05 0.95 0.95 0.05 — — —SrGa₂S₄:Eu (10% In) 0.05 0.95 0.90 0.10 — — — SrGa₂S₄:Eu (5% La) 0.050.95 0.95 — 0.05 — — SrGa₂S₄:Eu (5% Y) 0.05 0.95 0.95 — — 0.05 —

Phosphor Characterization

FIGS. 1 & 2 shows X-ray diffraction (XRD) data for SrGa₂S₄:Eu phosphor,referred to herein as NBG, and lithium, silicon co-doped NBG phosphor,according to some embodiments of the invention. The XRD patterns of thephosphors were measured using the Kα line of a Copper target. Data isshown for (1) NBG, (2) NBG plus lithium and 5 percent (%) silicon, and(3) NBG plus lithium and 10 percent silicon. (Here the percentages arederived from dividing the moles of silicon by the moles of gallium. Theamount of lithium in the phosphor is unknown, but clearly will notexceed the amount of starting material, and is assumed to provide chargebalance for the silicon dopant.) It is noted that the diffraction peaksare shifted only very slightly to larger angles with increase of dopantconcentration from 0 percent, to 5 percent, to 10 percent—the threesamples exhibit the same crystalline structure, with the shift to largerangle being consistent with Si being present in the SrGa₂S₄ crystallattice as a dopant.

The NBG and doped NBG phosphor particles (powder) were tested using anOcean Optics USB4000 spectrometer for photoluminescence intensity (PL)and chromaticity (CIE coordinates x and y). The results are summarizedin Table 7 for a range of different lithium and silicon dopantconcentrations. In the Table, PE is the wavelength at the peak of thephotoluminescence curve, PL is the relative intensity of thephotoluminescence peak, and FWHM is the full width at half maximum forthe photoluminescence peaks. The powder test method involves placing thephosphor powder on a stage, illuminating the phosphor powder with bluelight and measuring the emitted light; software is used to remove theblue light from the measurement to provide a measure of thephotoluminescence.

TABLE 7 Optical Properties of Li/Si doped SrGa₂S₄:Eu (5% Eu) - PowderTest FWHM Material PE (nm) PL CIE x CIE y (nm) SrGa₂S₄:Eu 536.0 0.910.2818 0.6799 49.2 SrGa₂S₄:Eu (Li/2.5% Si) 535.7 0.93 0.2804 0.6810 49.5SrGa₂S₄:Eu (Li/5% Si) 535.9 1.00 0.2811 0.6821 49.1 SrGa₂S₄:Eu (Li/7.5%Si) 536.2 1.04 0.2833 0.6809 48.8 SrGa₂S₄:Eu (Li/10% Si) 536.4 0.970.2847 0.6799 48.6

The NBG control sample and one of the Li/Si doped NBG samples weretested using a cavity test, and brightness, CIE x and CIE y weremeasured and quantum efficiency (QE) was calculated—results are providedin Table 8. The cavity test is similar to the powder test describedabove, except the phosphor powder is mixed with an uncurable encapsulantand placed in a cavity for testing, instead of the powder being placedon a stage, total emission is measured in an integrating sphere, and thebrightness includes blue light.

TABLE 8 Optical Properties of SrGa₂S₄:Eu (5% Eu) and Li/5% Si dopedSrGa₂S₄:Eu (5% Eu) (LED 459.6 nm) - Cavity Test Material Brightness (%)CIE x CIE y QE SrGa₂S₄:Eu 100 0.2070 0.3328 0.71 SrGa₂S₄:Eu (Li/5% Si)105 0.2074 0.3417 0.75

Table 9 provides powder test data for NBG phosphor and lithium, 10percent silicon co-doped NBG phosphor, according to some embodiments ofthe invention. FIG. 3 shows normalized emission spectra for the samesamples. Table 7 and FIG. 3 show that there is a trend to narrower FWHMfor the photoluminescence peaks with increasing lithium and silicondopant concentration over the tested range, although the peakphotoluminescence intensity appears to reach a maximum at aroundlithium/7.5% silicon.

TABLE 9 Optical Properties of SrGa₂S₄:Eu (5% Eu) and Li/10% Si dopedSrGa₂S₄:Eu (5% Eu) - Powder Test FWHM Material PE (nm) PL CIE x CIE y(nm) SrGa₂S₄:Eu 537.4 1.62 0.2931 0.6760 49.1 SrGa₂S₄:Eu (Li/10% Si)538.4 1.96 0.2937 0.6771 47.0

Tables 10, 11 & 12 provide powder test data and cavity test data for NBGphosphor doped with varying amounts of one or more of Ca, Ba, Li, In,and Si. The data shows the emission characteristics can be varied byusing these dopants. Furthermore, the Li/Si doped NBG phosphor showed animprovement over the un-doped NBG of the thermal properties of thephosphor as measured by the PL (Photoluminance) at 100° C. as apercentage of PL at room temperature, and the best thermal propertieswhen compared with the other doped samples.

TABLE 10 Optical Properties of Ca doped, Ba doped, Ca/Ba doped and Li/Sidoped SrGa₂S₄:Eu (5% Eu) - Powder Test PE PL FWHM % PL Material (nm) @25° C. CIE x CIE y (nm) @100° C. SrGa₂S₄:Eu 535.9 0.97 0.2819 0.681349.0 85.9 535.9 0.97 0.2815 0.6807 49.2 86.7 535.9 1.00 0.2815 0.681149.2 86.7 535.9 0.98 0.2816 0.6810 49.1 86.4 SrGa₂S₄:Eu (2% Ca) 537.11.06 0.2879 0.6790 48.4 85.3 537.1 1.04 0.2870 0.6793 48.6 84.6 537.01.09 0.2869 0.6791 48.7 86.6 537.1 1.06 0.2873 0.6791 48.6 85.5SrGa₂S₄:Eu (2% Ba) 536.3 1.01 0.2820 0.6821 48.5 84.4 535.9 1.07 0.28130.6826 48.6 86.0 535.6 1.01 0.2799 0.6823 49.0 86.9 535.6 1.03 0.28110.6823 48.7 85.8 SrGa₂S₄:Eu (4% Ca, 2% Ba) 537.0 1.06 0.2886 0.6777 48.984.8 537.0 1.00 0.2884 0.6766 49.2 85.5 537.0 1.08 0.2882 0.6777 49.087.3 537.0 1.05 0.2884 0.6773 49.0 85.9 SrGa₂S₄:Eu (Li/5% Si) 536.2 1.070.2832 0.6796 49.0 88.1 536.4 1.08 0.2850 0.6798 48.7 87.5 536.6 1.160.2859 0.6791 48.5 87.6 536.4 1.10 0.2847 0.6795 48.7 87.7

TABLE 11 Optical Properties of In doped, Li/Si doped and In, Li/Si dopedSrGa₂S₄:Eu (5% Eu) - Powder Test PE PL FWHM % PL Material (nm) @ 25° C.CIE x CIE y (nm) @100° C. SrGa₂S₄:Eu 535.7 0.68 0.2809 0.6810 49.2SrGa₂S₄:Eu 536.6 0.88 0.2836 0.6813 48.5 76.8 (2.5% In) SrGa₂S₄:Eu 536.50.89 0.2831 0.6814 48.6 73.8 (5% In) SrGa₂S₄:Eu 535.5 0.60 0.2802 0.680648.7 67.8 (10% In) SrGa₂S₄:Eu 536.2 1.05 0.2814 0.6827 48.6 (Li/2.5% Si)SrGa₂S₄:Eu 536.8 1.10 0.2848 0.6811 48.3 87.0 (Li/5% Si) SrGa₂S₄:Eu536.2 1.07 0.2848 0.6807 48.3 (Li/7.5% Si) SrGa₂S₄:Eu 536.8 0.89 0.28310.6814 48.6 77.7 (2.5% In, Li/2.5% Si)

TABLE 12 Optical Properties of Ba doped, Ca doped, Ca/Ba doped and Li/Sidoped SrGa₂S₄:Eu (5% Eu) - Cavity Test Material Brightness (%) CIE x CIEy QE SrGa₂S₄:Eu 97.2 0.2048 0.3285 0.69 SrGa₂S₄:Eu (2% Ba) 97.0 0.20410.3282 0.69 SrGa₂S₄:Eu (2% Ca) 103.7 0.2094 0.3383 0.73 SrGa₂S₄:Eu (4%Ca, 2% Ba) 101.0 0.2084 0.3344 0.71 SrGa₂S₄:Eu (Li/5% Si) 105.4 0.20790.3426 0.75

Tables 13-16 provide powder test data for NBG phosphor doped withvarying amounts of one or more of Ca, Al, Li, In, Si, Na, La and Y; theamount of Eu activator was also varied.

TABLE 13 Effect of Eu concentration in 2% Ca, 10% Al doped SrGa₂S₄:Eu -Powder Test PE PL FWHM % PL Material (nm) @ 25° C. CIE x CIE y (nm)@100° C. SrGa₂S₄:Eu (5%) 535.9 0.98 0.2816 0.6810 49.1 86.4 SrGa₂S₄:Eu(1% Eu, 2% Ca, 10% Al) 534.6 0.90 0.2798 0.6811 50.4 85.8 SrGa₂S₄:Eu (2%Eu, 2% Ca, 10% Al) 536.7 1.15 0.2842 0.6800 49.3 85.7 SrGa₂S₄:Eu (3% Eu,2% Ca, 10% Al) 537.5 1.21 0.2885 0.6766 49.0 84.2 SrGa₂S₄:Eu (4% Eu, 2%Ca, 10% Al) 538.3 1.19 0.2924 0.6752 49.1 83.5 SrGa₂S₄:Eu (5% Eu, 2% Ca,10% Al) 538.3 1.20 0.2932 0.6755 48.6 83.4

TABLE 14 Effect of Eu concentration in Li/5% Si doped SrGa₂S₄:Eu -Powder test PE PL FWHM % PL Material (nm) @ 25° C. CIE x CIE y (nm)@100° C. SrGa₂S₄:Eu 535.9 0.98 0.2816 0.6810 49.1 86.4 SrGa₂S₄:Eu 536.00.90 0.2801 0.6814 49.1 87.1 (1% Eu, Li/5% Si) SrGa₂S₄:Eu 537.1 1.090.2864 0.6776 48.2 86.7 (2% Eu, Li/5% Si) SrGa₂S₄:Eu 537.7 1.18 0.28920.6760 48.3 84.6 (3% Eu, Li/5% Si) SrGa₂S₄:Eu 538.5 1.14 0.2940 0.673147.6 85.3 (4% Eu, Li/5% Si) SrGa₂S₄:Eu 539.1 1.19 0.2974 0.6697 47.485.4 (5% Eu, Li/5% Si)

TABLE 15 Optical Properties of Na/Si doped, La doped, Y doped and Ca/Aldoped SrGa₂S₄:Eu (5% Eu) - Powder Test PE PL FWHM % PL Material (nm) @25° C. CIE x CIE y (nm) @100° C. SrGa₂S₄:Eu 535.9 0.98 0.2816 0.681049.1 86.4 SrGa₂S₄:Eu 537.6 0.91 0.2886 0.6792 47.5 81.8 (Na/5% Si)SrGa₂S₄:Eu 541.5 0.51 0.3307 0.6440 53.5 (5% La) SrGa₂S₄:Eu 537.6 1.330.2887 0.6782 48.5 84.0 (5% Y) SrGa₂S₄:Eu 537.4 1.22 0.2892 0.6776 49.186.5 (2% Ca, 10% Al)

TABLE 16 Optical Properties of Li/Si doped, Si doped, Ca/Al doped, Ydoped and Li/Y doped SrGa₂S₄:Eu (2% Eu) - Powder test PE PL FWHM % PLMaterial (nm) @ 25° C. CIE x CIE y (nm) @100° C. SrGa₂S₄:Eu 537.0 1.080.2837 0.6803 48.6 86.4 (Li/5% Si) SrGa₂S₄:Eu 536.4 1.06 0.2829 0.680449.1 86.0 (5% Si) SrGa₂S₄:Eu 537.7 1.08 0.2902 0.6757 49.5 84.5 (5% Ca,5% Al) SrGa₂S₄:Eu 536.6 0.99 0.2816 0.6798 49.5 84.6 (5% Y) SrGa₂S₄:Eu537.4 1.07 0.2879 0.6767 49.1 83.1 (5% Li, 5% Y)

FIG. 4 shows normalized peak emission versus temperature for NBGphosphor and NBG phosphor doped with various amounts of Li/Si, accordingto some embodiments of the invention. Li/Si doped NBG phosphor is shownto provide improved thermal properties compared to the un-doped NBG, andthe optimal doping amount was about 5% Si (measured as mole percent ofgallium).

Phosphor Coating

The NBG particles are coated by a CVD process in a fluidized bedreactor. FIG. 5 is a schematic representation of a phosphor particlecoating apparatus according to an embodiment of the invention. Reactor520 comprises a porous support disc 522, over which phosphor powder 524is held, and inlets 526 and 528 for metal organic (MO) precursor andwater (H₂O) vapor, respectively. The coating materials may be one ormore materials chosen from the group consisting of aluminum oxide,silicon oxide, titanium oxide, zinc oxide, magnesium oxide, zirconiumoxide and chromium oxide. The thickness may typically be in the range of100 nm (nanometers) to 5 μm (microns), in embodiments in the range of 50nm to 100 nm, 100 nm to 500 nm, 500 nm to 1 μm, or 1 μm to 2 μm. Herein,unless otherwise specified, coated NBG samples used in the examplesherein are coated with approximately 1 micron (μm) of alumina.

In a typical coating process, the phosphor powder sample was loaded intothe reactor and heated to 100-250° C., preferably 200° C., under N₂ gasflow. A metal oxide precursor such as TrimethylAluminum (TMA), Titaniumtetra-chloride (TiCl₄), Silicon tetra-chloride (SiCl₄), or DimethylZincwas introduced in to the reactor with a N₂ carrier gas through abubbler. H₂O vapor was also introduced into the reactor to react withthe metal oxide precursor to form oxide coating layers on phosphorparticles. Complete fluidization of the particles being coated (from gasflow optimization, etc.) without any dead space is important to ensurehomogeneous coating of all phosphor particles. In a typical coatingconducted at 200° C., for a 250 g phosphor particle loading of thereactor, the coating was produced with a metal oxide precursor feedingrate of 1 to 10 g/hour for 4 hours, while feeding H₂O at a rate of 2 to7 g/hour. It is shown below that these conditions can produce dense andsubstantially pinhole-free coatings of uniform thickness, with atheorized percentage solid space (percentage bulk density) of greaterthan 95% and in embodiments greater than 97% and in embodiments greaterthan 99%. In this patent specification, percentage solid space=(bulkdensity of the coating/density of the material within a singleparticle)×100. It will be understood that the percentage solid space (%solid space) provides a measure of the porosity of the coating resultingfrom pinholes. It is believed by the present inventors that outside of:the specified feeding rate range for oxide precursor, the specifiedfeeding rate range for H₂O, and/or the specified 100-250° C. temperaturerange, the coated phosphors may not exhibit the reliability documentedherein.

In the case of alumina coatings the inventors expect the coatings to bea dense amorphous oxide coating layer on the NBG phosphor particlesurface without pinholes (pinhole-free) that is, a dense waterimpermeable coating.

Characterization of Coated Phosphor

FIGS. 6A-6E show SEM micrographs of alumina coated NBG phosphorparticles, according to an embodiment of the invention. In FIGS. 6D and6E a cross-section is shown where phosphor particles 630 are seen coatedwith a uniform dense impermeable (pinhole-free) coating of alumina 632of approximately one micron in thickness—a variation in thickness wasmeasured over the range of 0.87 μm to 1.17 μm; the particles are mountedin epoxy 634 for making the cross-sections. The samples were prepared bydispersing the phosphor particles in epoxy then curing. After curing,the epoxy (with coated NBG powder) was polished and then covered by aflash of sputtered Pd—Au metal to enhance the sample's electronicconductivity (the metal reduces/removes electron charging when analyzingthe sample in the SEM). The prepared cross section sample was thenanalyzed by scanning electron microscopy (SEM) which clearly shows thehermetic coating layer of alumina around the NBG particles (completecoverage by coating layer of particle without observable gaps orpinholes), as shown in FIGS. 6D & 6F.

FIGS. 7A-7D show SEM micrographs of M535 phosphor particles—M535 is acoated alkaline earth metal thiogallate phosphor available from DowChemical. In FIGS. 7C and 7D a cross-section is shown where phosphorparticles 730 are seen coated with a coating 732 of approximately 0.4 μmin thickness—a variation in thickness was measured over the range ofless than 0.1 μm (100 nm) to 0.5 μm (500 nm); the particles are mountedin epoxy 734 for making the cross-sections. The SEM samples wereprepared as described above. FIGS. 7C & 7D clearly show a non-uniformcoating layer around the M535 particles (incomplete coverage by coatinglayer of particle with observable gaps and pinholes).

The stability and reliability of the coated NBG phosphor particles ofthe present invention may be established using a silver test, asfollows. Silver ions (Ag⁺) can attack sulfur in NBG phosphors to form ablack Ag₂S compound if the NBG surface is not well protected (forexample, if pinholes are present in the coating then black Ag₂S spotswould form). The silver test is based on this mechanism and involvessoaking the coated NBG materials in AgNO₃ solution to evaluate how wellthe coating layer is able to protect the NBG phosphor particle againstAg⁺ attack. The longer the time the NBG phosphor can survive in the Agtest, the better the surface protection (coating/reliability) thephosphor has.

In a Ag test, NBG powder was soaked in 1 mole/liter AgNO₃ solution, andthe stability of the sample was evaluated by monitoring how long thepowder can survive without turning black at room temperature, 50° C. and85° C. For comparison, it is noted that uncoated NBG samples turn blackin less than one minute. Test results show that a coated sample,prepared as described herein, can survive without blackening for morethan: 24 hours at room temperature; 24 hours at 50° C.; and 3 hours at85° C.

The stability and reliability of the coated NBG phosphor particles ofthe present invention may also be established using an electricalconductivity test. In this test uncoated and coated NBG particles aredispersed in de-ionized water and the electrical conductivity ismonitored over time—increases in conductivity being attributed torelease of ions into solution from the particles. An Oakton waterproofCON 150 meter was used for these tests. Samples were 0.1 g of phosphorparticles dispersed in 10 ml of distilled water. Results for aluminacoated NBG and uncoated NBG are shown in Table 17. The alumina coatedphosphor clearly performs much better in this test, indicating thealumina coating is providing good protection of the phosphor from attackby water and dissolution.

TABLE 17 Electrical conductivity versus time for SrGa₂S₄:Eu (5% Eu) andalumina (Al₂O₃) coated for SrGa₂S₄:Eu (5%) - 0.1 g phosphor dispersed in10 ml de-ionized water Electrical Conductivity @ 25° C. (μs) 0 0.5 1 2 34 Material hour hour hour hour hour hour SrGa₂S₄:Eu 8.6 38.7 83.0 216.0219.0 218.0 Alumina coated SrGa₂S₄:Eu 1.4 9.2 10.8 11.6 12.1 12.0

The coated phosphor samples are also tested in packaged form, referredto herein as package test. The coated NBG phosphor is mixed with otherphosphors in an optical encapsulant, such as Phenylsiloxane resin OE6630available from Dow Corning. The mixture of phosphors and encapsulant areloaded in an LED package, such as SMD 7020 LED module. The encapsulantis cured and optical measurements are made, as described below.Furthermore, the packaged phosphors are subjected to accelerated testingconditions, known as “wet high temperature operating life” testing andreferred to herein as WHTOL—the testing requires exposure to 85° C./85%RH (relative humidity) while operating at 120 mA.

FIG. 8 shows reliability data, relative photoluminescence intensityversus time, for an LED (SMD 7020 LED module 450.0-452.5 nm) operatedunder accelerated testing conditions WHTOL 85° C./85% RH for (i) aluminacoated NBG phosphor, according to an embodiment of the invention, (ii)M535 phosphor, and (iii) β-SiAlON:Eu (540 nm) phosphor. FIG. 9 showsreliability data, change of chromaticity ΔCIE x versus time, for thesame samples as in FIG. 8. FIG. 10 shows reliability data, change ofchromaticity ΔCIE y versus time, for the same samples as in FIGS. 8 & 9.The figures show the brightness and CIE of the white LED package withthe alumina coated NBG phosphor of the present invention is as stable asthe control package with β-SiAlON:Eu phosphor, and exhibits much betterstability than the package with M535.

FIGS. 11A-11C show SEM micrographs of multi-layer coated(alumina—aluminum oxide—Al₂O₃, titania—titanium dioxide—TiO₂) NBGphosphor particles, according to an embodiment of the invention. InFIGS. 11B and 11C a cross-section is shown where phosphor particles 1130are seen coated with a uniform pinhole-free (impermeable) coating ofalumina 1132 of approximately 0.5 μm in thickness—a variation inthickness was measured cover the range of 0.46 μm to 0.62 μm—followed bya second coating of titania 1136 of approximately 0.1 μm in thickness;the particles are mounted in epoxy 1134 for making the cross-sections.The SEM samples were prepared as described above. FIGS. 11B & 11Cclearly show the hermetic coating layer of alumina around the NBGparticles (complete coverage by coating layer of particle withoutobservable gaps or pinholes). WHTOL testing under 60° C./90% RH ofpackages with alumina/titania coated NBG phosphor particles showsimproved performance over packages with alumina coated NBG phosphorparticles when combined with KSF narrow band red phosphor particles; thedata suggests that the titania layer over the alumina coating mayimprove the protection by the coating of the phosphor in a humidenvironment.

FIG. 12 shows reliability data, relative photoluminescence intensityversus time, for a white light emitting device (SMD 7020 LED module)operated under accelerated testing conditions WHTOL 85° C./85% RH for ablue LED (450.0-452.5 nm) combined with (i) alumina coated NBG phosphorand CSS red phosphor (alumina coated CaSe_(1-x)S_(x):Eu; 630 nm),according to an embodiment of the invention, and (ii) β-SiAlON:Euphosphor (540 nm) and CSS red phosphor (630 nm). FIG. 13 showsreliability data, change of chromaticity ΔCIE x versus time, for thesame samples as in FIG. 12. FIG. 14 shows reliability data, change ofchromaticity ΔCIE y versus time, for the same samples as in FIGS. 12 &13.

FIG. 15 shows reliability data, relative photoluminescence intensityversus time, for a white light emitting device (SMD 7020 LED module)operated under accelerated testing conditions WHTOL 60° C./90% RH for ablue LED (450.0-452.5 nm) combined with (i) alumina coated NBG phosphorand KSF (K₂SiF₆:Mn) red phosphor, according to an embodiment of theinvention, ii) alumina coated NBG phosphor and CSS red phosphor (630nm), according to an embodiment of the invention, iii) β-SiAlON:Eu (540nm) phosphor and KSF red phosphor, and (iv) β-SiAlON:Eu (540 nm)phosphor and CSS red phosphor (630 nm). FIG. 16 shows reliability data,change of chromaticity ΔCIE x versus time, for the same samples as inFIG. 15. FIG. 17 shows reliability data, change of chromaticity ΔCIE yversus time, for the same samples as in FIGS. 15 & 16. The performancedata for the 60° C./90% RH test—an industry alternative to the 85°C./85% RH test—is even better than for the 85° C./85% RH test.

Packaged White Light Emitting Device, for Display Backlight and GeneralLighting Device

FIG. 18 illustrates a white light emitting device, according to someembodiments. The device 1800 can comprise a blue light emitting, withinthe range of 450 nm to 470 nm, GaN (gallium nitride) LED chip 1802, forexample, housed within a package. The package, which can for examplecomprise a low temperature co-fired ceramic (LTCC) or high temperaturepolymer, comprises upper and lower body parts 1804, 1806. The upper bodypart 1804 defines a recess 1808, often circular in shape, which isconfigured to receive the LED chips 1802. The package further compriseselectrical connectors 1810 and 1812 that also define correspondingelectrode contact pads 1814 and 1816 on the floor of the recess 1808.Using adhesive or solder, the LED chip 1802 can be mounted to athermally conductive pad 1818 located on the floor of the recess 1808.The LED chip's electrode pads are electrically connected tocorresponding electrode contact pads 1814 and 1816 on the floor of thepackage using bond wires 1820 and 1822 and the recess 1808 is completelyfilled with a transparent polymer material 1822, typically a silicone,which is loaded with a mixture 1824 of a green phosphor and a redphosphor material of the present invention such that the exposedsurfaces of the LED chip 1802 are covered by the phosphor/polymermaterial mixture. To enhance the emission brightness of the device thewalls of the recess are inclined and have a light reflective surface.

As shown in Tables 18 & 19, the alumina coated NBG phosphor samples ofthe present invention exhibit higher NTSC and brightness compared toβ-SiAlON:Eu phosphor.

TABLE 18 Optical Properties of white light emitting devices (backlights)with a Hisense TV color filter - Cavity Test Device 1: LED (452.0 nm) +alumina coated NBG (537 nm) + KSF Device 2: LED (452.0 nm) + β-SiAlON:Eu(535 nm) + KSF Phosphor Green Red per 100 g phosphor phosphor FluxBrightness Device silicone (g) (%) (%) (lm) (%) CIE x CIE y 1 55 14.086.0 44.07 100.1 0.2805 0.2607 2 55 23.8 76.2 44.01 100.0 0.2805 0.2607

TABLE 19 Optical Properties of white light emitting devices (backlights)with a Hisense TV color filter - Cavity Test Device 3: LED (455.1 nm) +alumina coated NBG (537 nm) + KSF Device 4: LED (455.1 nm) + aluminacoated NBG (5% Ca, 540 nm) + KSF Phosphor Green Red per 100 g phosphorphosphor Flux Brightness Device silicone (g) (%) (%) (lm) (%) CIE x CIEy 3 18 18.2 81.8 4.86 100.0 0.2806 0.2616 4 18 15.9 84.1 4.93 101.50.2806 0.2616

FIG. 19 shows a white light emission spectra of a 3000K white lightemitting device comprising alumina coated NBG phosphor (537 nm) and CSSred phosphor (626 nm), according to some embodiments of the invention.

In addition to its applications in general LED lighting applications,due to its narrow band green and suitable wavelength, NBG phosphors canalso be used in display backlighting. FIG. 20 shows the filteringcharacteristics, light transmission versus wavelength, for red, greenand blue filter elements of a Hisense filter of an LCD display optimizedfor TV applications. FIGS. 21 & 22 show alumina coated NBG phosphor (537nm), according to some embodiments, with red KSF phosphor, before andafter filtering, respectively, which shows separation of blue, green andred peaks. Table 20 shows, when used with KSF, alumina coated NBGphosphor with an emission wavelength of about 537 nm can achieve 92% ofthe area of the NTSC standard. Note that the LCD white measurements arefor an LCD operating to produce a white screen and using a backlight LEDaccording to embodiments, and the LCD red/green/blue filter measurementsare for light from the LCD which comes only through the particular colorfilter—red, green or blue.

TABLE 20 Optical Properties of white light emitting device:LED (452.0nm) + alumina coated NBG (537 nm) + KSF - Hisense TV Color FilterParameter Value Backlight LED CIE x 0.2805 Backlight LED CIE y 0.2608Backlight LED Brightness (lm) 44.1 LCD white CIE x 0.2865 LCD white CIEy 0.3080 LCD Brightness (lm) 43.7 Brightness LCD/LED (%) 99.1 Red CIE xafter LCD red filter 0.6806 Red CIE y after LCD red filter 0.3041 GreenCIE x after LCD green filter 0.2442 Green CIE y after LCD green filter0.6605 Blue CIE x after LCD blue filter 0.1507 Blue CIE y after LCD bluefilter 0.0691 NTSC (%) 92.1FIG. 23 shows the filtering characteristics, light transmission versuswavelength, for red, green and blue filter elements of an AUO filter ofan LCD display. (The AUO color filter is a thin filter often used inlaptop and monitor applications.) FIGS. 21 & 24 show alumina coated NBGphosphor (537 nm), according to some embodiments, with red KSF phosphor,before and after filtering, respectively, which shows separation ofblue, green and red peaks. Table 21 shows, when used with KSF, aluminacoated NBG phosphor with an emission wavelength of about 537 nm canachieve 102% of the area of the NTSC standard.

TABLE 21 Optical Properties of white light emitting device:LED (452.0nm) + alumina coated NBG (537 nm) + KSF - ALO (AU Optronics Corp) ColorFilter Parameter Value Backlight LED CIE x 0.2805 Backlight LED CIE y0.2608 Backlight LED Brightness (lm) 44.1 LCD white CIE x 0.3272 LCDwhite CIE y 0.3391 LCD Brightness (lm) 36.5 Brightness LCD/LED (%) 102.6Red CIE x after LCD red filter 0.6917 Red CIE y after LCD red filter0.3081 Green CIE x after LCD green filter 0.2319 Green CIE y after LCDgreen filter 0.6843 Blue CIE x after LCD blue filter 0.1515 Blue CIE yafter LCD blue filter 0.0439 NTSC (%) 102.6

White LEDs using combined blue LED and YAG:Ce phosphor have been widelyused as backlights for personal computer LCD screens, LCD TVs andsmall-sized LCDs used in devices such as cellular phones and tabletdisplays. To date, the color gamut of these LEDs can attainapproximately 70% of the area of the NTSC standard, and the widest colorgamut using a narrow-band β-SiAlON:Eu green phosphor and CaAlSiN₃:Eu redphosphor can reach ˜85% of the area of the NTSC standard with theassistance of typical LCD color filters. However, the combination of acoated NBG phosphor, as described herein, with an emission wavelength ofabout 537 nm, with a narrow band red phosphor, such as KSF, can reachapproximately 92% of the area of the NTSC standard with a TV colorfilter. See FIG. 25 which shows the 1931 CIE color coordinates of theNTSC standard (callout 2510) and the calculated RGB coordinates from awhite light source comprising a blue LED (451 nm) combined with thecoated NBG phosphor of the present invention with the red narrow bandphosphor KSF (callout 2530); this is the same white light source forwhich a spectrum is shown in FIG. 21 and described above in Table 20.Note that herein references to the percentage of the area of the NTSCstandard are percentages of the area of the NTSC (National TelevisionSystem Committee) 1953 color gamut specification as mapped on the CIE1931 xy chromaticity diagram. Note that the combination of a β-SiAlON:Eugreen phosphor and a KSF red phosphor is provided for comparison, and isseen to attain 89% of the area of the NTSC standard (callout 2520) witha TV color filter.

It is expected that some embodiments of the coated NBG phosphors of thepresent invention, when combined with one of the various possible narrowband red phosphors such as KSF or CSS are able to reach highefficiencies and high levels of color gamut for LED backlightapplications, where the phosphors are integrated into “on-chip”,“on-edge” or “on-film” LED backlights. Furthermore, it is expected thatthe performance of some embodiments of the coated narrow band greenphosphors of the present invention in combination with one of thevarious possible narrow band red phosphors will provide higherefficiencies, better color purity and higher levels of color gamutcompared with prior art phosphor combinations.

Remote Phosphor White Light Emitting Device

FIGS. 26A and 26B illustrate a remote phosphor solid-state white lightemitting device, according to some embodiments. The device 2600 isconfigured to generate warm white light with a CCT (Correlated ColorTemperature) of 2700K and a CRI (Color Rendering Index) of about 90. Thedevice can be used as a part of a downlight or other lighting fixture.The device 2600 comprises a hollow cylindrical body 2602 composed of acircular disc-shaped base 2604, a hollow cylindrical wall portion 2606and a detachable annular top 2608. To aid in the dissipation of heat,the base 2604 is preferably fabricated from aluminum, an alloy ofaluminum or any material with a high thermal conductivity. The base 2604can be attached to the wall portion 2606 by screws or bolts or by otherfasteners or by means of an adhesive.

The device 2600 further comprises a plurality (four in the exampleillustrated) of blue light emitting LEDs 2612 (blue LEDs) that aremounted in thermal communication with a circular-shaped MCPCB (metalcore printed circuit board) 2614. The blue LEDs 2612 can comprise aceramic packaged array of twelve 0.4 W GaN-based (gallium nitride-based)blue LED chips that are configured as a rectangular array 3 rows by 4columns. To maximize the emission of light, the device 2600 can furthercomprise light reflective surfaces 2616 and 2618 that respectively coverthe face of the MCPCB 2614 and the inner curved surface of the top 2608.

The device 2600 further comprises a photoluminescent wavelengthconversion component 2620 that is located remotely to the LEDs andoperable to absorb a proportion of the blue light generated by the LEDs2612 and convert it to light of a different wavelength by a process ofphotoluminescence. The emission product of the device 2600 comprises thecombined light generated by the LEDs 2612 and the photoluminescentwavelength conversion component 2620. The photoluminescent wavelengthconversion component may be formed of a light transmissive material (forexample, polycarbonate, acrylic material, silicone material, etc.) andcomprises a mixture of a yellow, red and/or green phosphor, includingcoated NBG phosphor material of the present invention. Furthermore, inembodiments the photoluminescent wavelength conversion component may beformed of a light transmissive material coated with one or more layersof phosphor materials as described above, including coated greenphosphor material of the present invention. The wavelength conversioncomponent is positioned remotely to the LEDs 2612 and is spatiallyseparated from the LEDs. In this patent specification “remotely” and“remote” means in a spaced or separated relationship. The wavelengthconversion component 2620 is configured to completely cover the housingopening such that all light emitted by the lamp passes through thecomponent 2620. As shown the wavelength conversion component 2620 can bedetachably mounted to the top of the wall portion 2606 using the top2608 enabling the component and emission color of the lamp to be readilychanged.

While in the above embodiment the phosphors are located in thephotoluminescent wavelength conversion component it is contemplated inother embodiments to locate one of the phosphors in the photoluminescentwavelength conversion component and the other within a package housingthe LEDs

Although examples of the present invention have been described primarilywith reference to NBG phosphor particles coated with a single material,in certain embodiments, it is envisaged that the coatings comprisemultiple layers (two, three or more) with combinations of the coatingmaterials described herein. Furthermore, the combination coatings may becoatings with an abrupt transition between adjacent coating materials,or may be coatings in which there is a gradual transition from onecoating material to another coating material thus forming a zone withmixed composition that varies through the thickness of the coating.

Although the present invention has been particularly described withreference to certain embodiments thereof, it should be readily apparentto those of ordinary skill in the art that changes and modifications inthe form and details may be made without departing from the spirit andscope of the invention.

What is claimed is:
 1. A coated phosphor comprising: phosphor particlescomprised of a phosphor material with composition (M)(A)₂S₄:Eu, wherein:M is at least one of Mg, Ca, Sr and Ba; and A is at least one of Ga, Al,In, Y; and a dense impermeable coating of an oxide materialencapsulating individual ones of said phosphor particles; and whereinsaid coated phosphor is configured to satisfy at least one of theconditions: (1) under excitation by blue light, the reduction inphotoluminescent intensity at the peak emission wavelength after 1,000hours of aging at about 85° C. and about 85% relative humidity is nogreater than about 30%; (2) the change in chromaticity coordinatesCIE(y), ΔCIE y, after 1,000 hours of aging at about 85° C. and about 85%relative humidity is less than about 5×10⁻³; (3) said coated phosphordoes not turn black when suspended in a 1 mol/L silver nitrate solutionfor at least two hours at 85° C.; (4) said coated phosphor does not turnblack when suspended in a 1 mol/L silver nitrate solution for at leastone day at 20° C.; and (5) said coated phosphor does not turn black whensuspended in a 1 mol/L silver nitrate solution for at least 5 days at20° C.
 2. The coated phosphor of claim 1, wherein said coated phosphoris configured such that the change in chromaticity coordinates CIE(y),ΔCIE y, after 1,000 hours of aging at about 85° C. and about 85%relative humidity is less than or equal to about 3×10⁻³.
 3. The coatedphosphor of claim 1 or claim 2, wherein said oxide is one or morematerials chosen from the group comprising aluminum oxide, siliconoxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide andchromium oxide.
 4. The coated phosphor of claim 1 or claim 2, whereinsaid oxide material is alumina.
 5. The coated phosphor of any one ofclaims 1 to 4, wherein said coating has a thickness in the range of 100nm to 5 μm.
 6. The coated phosphor of any one of claims 1 to 5, whereinsaid coating has a thickness in the range of 800 nm to 1.2 μm.
 7. Thecoated phosphor of any one of claims 1 to 6, wherein said phosphorparticles have a size distribution having a D₅₀ value within the rangeof 5 μm to 15 μm.
 8. The coated phosphor of any one of claims 1 to 7,wherein M is Sr.
 9. The coated phosphor of any one of claims 1 to 8,wherein A is Ga.
 10. The coated phosphor of any one of claims 1, 2 and 5to 7, wherein A is Ga, M is Sr and said oxide material is alumina. 11.The coated phosphor of any one of claims 1 to 10, wherein said coatedphosphor has a peak photoluminescence between 535 nm and 537 nm and aFWHM of between 48 nm and 50 nm, when excited by blue light with a peakemission of about 450 nm.
 12. The coated phosphor of any one of claims 1to 11, wherein the phosphor material composition further comprises Pr.13. A method of forming a coated phosphor, comprising: providingphosphor particles comprised of a phosphor material with composition(M)(A)₂S₄:Eu wherein: M is at least one of Mg, Ca, Sr and Ba; and A isat least one of Ga, Al, In, Y; and depositing a dense impermeablecoating of an oxide material encapsulating individual ones of saidphosphor particles by a gas phase process in a fluidized bed reactor;wherein said coated phosphor is configured to satisfy at least one ofthe conditions: (1) under excitation by blue light, the reduction inphotoluminescent intensity at the peak emission wavelength after 1,000hours of aging at about 85° C. and about 85% relative humidity is nogreater than about 30%; (2) the change in chromaticity coordinatesCIE(y), ΔCIE y, after 1,000 hours of aging at about 85° C. and about 85%relative humidity is less than about 5×10⁻³; (3) said coated phosphordoes not turn black when suspended in a 1 mol/L silver nitrate solutionfor at least two hours at 85° C.; (4) said coated phosphor does not turnblack when suspended in a 1 mol/L silver nitrate solution for at leastone day at 20° C.; and (5) said coated phosphor does not turn black whensuspended in a 1 mol/L silver nitrate solution for at least 5 days at20° C.
 14. The method of claim 13, wherein said coating has a thicknessin the range of 100 nm to 5 μm.
 15. The method of claim 13 or claim 14,wherein said coating has a thickness in the range of 800 nm to 1.2 μm.16. The method of any one of claims 13 to 15, wherein said phosphorparticles have a size distribution having a D₅₀ value within the rangeof 5 μm to 15 μm.
 17. The method of any one of claims 13 to 16, whereinsaid oxide material is one or more materials chosen from the groupcomprising aluminum oxide, silicon oxide, titanium oxide, zinc oxide,magnesium oxide, zirconium oxide and chromium oxide.
 18. The method ofany one of claims 13 to 16, wherein M is Sr and A is Ga and said oxidematerial is alumina.
 19. The method of any one of claims 13 to 18,wherein said coated phosphor has a peak photoluminescence between 530 nmand 545 nm and a FWHM of between 45 nm and 60 nm, when excited by bluelight with a peak emission of about 450 nm.
 20. A white light emittingdevice comprising: an excitation source with a dominant emissionwavelength within a range from 200 nm to 480 nm; a coated phosphor as inany one of claims 1 to 12, with a first phosphor peak emissionwavelength; and a second phosphor with a second phosphor peak emissionwavelength different to said first phosphor peak wavelength.
 21. Thewhite light emitting device of claim 20, wherein said coated phosphorabsorbs radiation at a wavelength of 450 nm and emits light with aphotoluminescence peak emission wavelength between about 530 nm andabout 545 nm; and said second phosphor emits light with aphotoluminescence peak emission wavelength between about 600 nm andabout 650 nm.
 22. The white light emitting device of claim 21, whereinthe excitation source has a dominant emission wavelength within a rangefrom 440 nm to 480 nm; and wherein said white light emitting device hasan emission spectrum with clearly separated blue, green and red peaks,and a color gamut after LCD RGB color filters of at least 85% of NTSC.23. The white light emitting device of any one of claims 20 to 22,wherein at least one of the coated phosphor and second phosphor islocated in a remote phosphor component.
 24. The white light emittingdevice of claim 23, wherein the coated phosphor is located in the remotephosphor component and the second phosphor is located in a packagehousing the excitation source.
 25. The white light emitting device ofclaim 23 or claim 24, wherein the remote phosphor component comprises afilm.